The invention concerns a process for the continuous production of metal alloys.
The production of alloys in the foundry involves a number of technological requirements which can not be fully satisfied by the present state of the art. The product of the process should later satisfy high demands with respect to homogeneity, and should contain as little as possible of non-metallic impurities which can be picked up at various stages. During the whole of the time that alloy additions are being made, the device for making these additions is required to exhibit a calculated dosage accuracy of .+-.0.2 to 2%. Besides this there should be as little as possible loss of metal due to dross formation and combustion of alloying elements. From the point of view of economy it is necessary that the process can be automated easily so that it can be operated with minimum time consumption, under the most favorable conditions of job hygiene, and with the minimum loss of material due to starting and stopping procedures.
In the present state of the art the problem of alloying is solved mainly by stirring, by which is to be understood the production of a relative movement between the two components to be mixed using mechanical forces, where both components are in motion with respect to the stirring system and the mechanical forces can be produced by a moveable stirrer or by a gas being blown through the melt. If this mechanical stirring is carried out in a batch type process, there are a number of disadvantages experienced.
Mechanical stirring devices are relatively susceptible to wear and therefore involve high maintenance costs. In many furnace lines there is a shortage of space and so the mechanical stirring must be done by hand. Since the effectiveness of the process is then to a large extent dependent on the attention and care exerted by the individual foundry worker, and because the work itself is found to be unpleasant physiologically and gives rise to doubts with respect to job hygiene, wrong compositions are produced and unplanned delays in production procedure arise because of the need for alloy adjustments to be made. On the other hand, if the melt is stirred by passing a gas through it, the porous ceramic block needed for this must be built in to the melt container, or lances must be used, both devices being of the kind which are particularly susceptible to wear. Mechanical stirring, in particular by flushing the melt with a gas, causes additional dross to be formed, and in unfavorable cases this dross can be rich in alloying elements. In addition to the alloying elements which are purposefully added to the melt, mechanical stirring is responsible for the introduction of undesireable non-metallic inclusions in the form of oxides, for example, which become uniformly distributed throughout the melt. These inclusions give rise to problems of material quality both during and after further processing as they cause grey streaks, tool wear and foil porosity. Stirring in alloying elements mechanically also leads to crust formation at the furnace walls, which consequently increases maintenance costs. The most serious disadvantage is that, when alloying additions of elements such as Mn, Ti, Sr, Fe etc. are made by mechanical stirring, the required degree of homogeneity (efficiency of mixing) is not achieved, so that the longer route involving expensive master alloys has to be taken (see for example Aluminum Master Alloys DIN 1725 Sheet 3, June 1973; H. Nielsen (Hg) Aluminium-Taschenbuch, 13th edition, Dusseldorf, 1974, pages 12-14).
In the above mentioned mechanical stirring processes, the mixing action for the requisite relative movement is produced by moving stirring elements which transfer their energy to the components being mixed. The static mixer on the other hand employs a relative movement whereby fixed mixing elements act as obstacles, and the components to be mixed derive their energy of movement from a delivery facility which overcomes the pressure loss in the mixer. Static mixers representing the present state of the art comprise a system of tubes with a row of such static mixing elements which produce the mixing effect by repeated division and displacement of the component streams. Such a static mixer can be characterized by the homogeneity (efficiency of mixing of the mixed product, the pressure loss in the melt container system and by the considerable heat transfer which possibly occurs (see Bruenemann/John, Chemie-Ing.-Technik, 43 (1971), 348, and with particular reference to heat transfer J. Gomori, Chemie-Ing.-Technik, 49 (1977), 39-40).
Static mixers are especially suitable for the continuous mixing of very viscous or aggressive fluids, either mixing them together or mixing with solids. They have however proved to be particularly good in the special field of mixing gas streams, for example in the technology of air conditioning, in the centers of hot and cold testing facilities, and in plants for drying a wide variety of products (J. Gomori, Static mixing of gas streams, Chemie-Ing.-Technik, 49 (1977), 39-40). The present state of the art is such that the stationary elements split or divide up the liquid or gas streams, divert the distributary streams and unite them again, as a result of which layers of material of changing composition are produced, their number increasing with the number of displacement elements employed. Theoretically, by means of appropriate choice of element and in particular by maximizing their number within temporary limiting conditions, any required degree of mixing can be achieved.
Static mixers representing the present state of the art have no moving parts; the pressure loss which the melt suffers in the mixer has to be overcome by the facility delivering the melt. The requisite work of mixing is then--besides other things--provided by the reduction in the kinetic energy of the stream of material which expresses itself by the mixture suffering a corresponding loss of pressure and velocity (J. Gomori, ibid, O. A. Pattison, Motionless Inline Mixers, Chem., Eng., 1969, (5), following p. 94; T. Bor, The Static Mixer as a Chemical Reactor, Brit. Chem. Eng. 1971, pages 610-612; H. Bruenemann/G. John, Efficiency of Mixing and Pressure Losses in Static Mixers of Various Design, Chemie-Ing.-Technik 43 (1971), pages 348-356; Ullmann's Encyclopaedia of Techn.Chem. 4.A. 1972, Vol.2, follow. p. 267).
Among the disclosed versions of static mixers representing the state of the art there are some which are not suitable for the preparation of alloys as the transport of molten metals in closed pipe systems presents additional technical problems. If use is made of a mixer with closed flow channels, in which the pressure at the entry to the mixer is produced by conventional pumps and the connection between the flow channels and the displacement elements is permanent, then there is a danger that the device will become blocked due to the displacement elements being permanently anchored in the through-flow channel. Maximizing the number of displacement elements, a feature which seems desireable to optimize the efficiency of mixing actually exacerbates this situation considerably (U.S. Pat. No. 2,894,732 of Shell Co., 3,051,452, 3,051,453 and 3,182,965, 3,206,178 of American Enka Co., U.S. Pat. No. 3,195,865 of the Dow Badische Co.).
In the mixer with closed flow channels there is a large pressure loss as a result of the friction between the deflection elements and the components being mixed. The more displacement elements there are in the system the more pronounced is the pressure drop between the entry and exit points in the mixer. In a favorable case the pressure drop in the static mixer is four times that produced by a comparable empty flow channel (O. A. Pattison ibid p. 95), with the result that the pressure drop has to be overcome by a suitable delivery system.
In mixers with closed flow channels and permanently installed displacement elements the latter are not readily accessable and are therefore difficult to clean mechanically. This can lead to a greater danger of corrosion and therefore to a shorter service life. If the mixture contains expensive ingredients then for the same reason the resultant loss of material becomes an important factor. Such a loss is greater the more displacement elements in the system, the desired number of elements being determined by other considerations.
Finally, the normal design of static mixer requires a relatively complicated geometry of displacement and mixing elements in order to avoid so called tunneling of the mixture components. By "tunneling" here is to be understood coarse inhomogeneities in the product in the form of a breakthrough of an individual component of the mixture (Bruene-mann/John, ibid p. 352). In one of the common versions of the static mixture this problem has led to the practice that one or more alternating left and right hand displacement elements are in the form of perforated sheet and are arranged in series one behind the other (O. A. Pattison, ibid p. 95). The version of mixer described in U.S. Pat. No. 3,195,865 contains mixing and displacement elements of a particularly complicated geometry. Such complicated geometrical arrangements incur high assembly costs which are raised further by the fact that the mechanical properties of the junction between displacement element and flow channel have to meet high standards in order that compensation can be made for the relatively large pressure difference.